The present disclosure relates to semiconductor processing methods, and particularly to methods for anisotropically etching silicon nitride with high selectivity to silicon and silicon oxide, and structures for effecting the same.
Etching silicon nitride selective to more than one material is very challenging. Poor performance in selective etching of silicon nitride is often observed especially at the nanoscale level.
An example of poor performance in selectivity of a nitride etch process is observed in the spacer module in which silicon nitride spacers are formed to electrically isolate gate electrodes from source and drain regions. Lack of sufficient selectivity causes formation of recesses within semiconductor material portions that are not covered by the gate electrodes.
Another example of poor performance in selectivity of a nitride etch is observed in a trench silicide etch in which a silicon nitride layer is employed as an etch stop layer. Lack of sufficient selectivity during the etch of the silicon nitride layer causes a dielectric layer to be excessively etched or recess formed in the silicide itself.
A typical silicon nitride etch process employs single carbon CHxFy gases which are admixed with Ar, H2, N2 and/or O2 gases. A fluorohydrocarbon plasma employed in such a silicon nitride etch process is selective to silicon oxide, i.e., does not etch silicon oxide. Selectivity to silicon is facilitated by admixture of the O2 gas, which converts silicon into silicon oxide and thus, prevents further erosion of silicon upon formation of a silicon oxide layer. Variations of the silicon etch process are employed in such modules such as a trench silicide module.
One of the drawbacks of the silicon nitride etch employing CH3F and O2 gases is that the selectivity to silicon is not inherent in the etch process, and that the selectivity to silicon relies on conversion of silicon to silicon oxide. At the nanoscale level, however, the conversion of silicon into silicon oxide requires significant consumption of silicon on a relative scale. As a result, the silicon nitride etch process known in the art does not provide high selectivity to silicon at the nanoscale level.